How do I unlock my phone?

Not sure if this is the place to post this but here goes. I upgraded my old Captivate to a Galaxy S3. My provider is AT&T. I would like to be able to buy a SIM card from a pay as you go provider and use it in the Captivate. How do I do this? Does the phone need to be unlocked and if so how do I do that?

First off you need to find out if the captive will work with the pay as you go plan you might not have to unlock it at all. If that's not the case take the phone to the pay as you go provider and ask if they unlock phone. Alot of providers will unlock for free if they can.